Modeling of fatigue behavior based on interaction between time- and cyclic-dependent mechanical properties

A general constitutive equation was established using the theory of viscoelasticity in order to consider the interaction of the time- and cyclic-dependent mechanical properties of laminated composites. This equation was solved for two specific loading patterns, (1) stress unloading to zero stress level (recovery solution), and (2) load control sinusoidal loading (fatigue solution), and was subsequently imported to model tensile-tensile interrupted fatigue experiments (including recovery phases) of +/- 45 degrees angle-ply glass/epoxy composite laminates at different stress levels. The viscoelastic parameters in the recovery solution were calibrated at different percentages of fatigue lifetime using the experimental recovery results. The estimated viscoelastic parameters were then imported into the fatigue solution to predict the fatigue stiffness, hysteresis loop area, cyclic creep, storage and loss moduli as well as tan(delta) under cyclic loading. The theoretical predictions compared well to the experimental data.